1. Field of Invention
The invention relates to broadband communication networks.
2. Prior Art
Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme used in communication systems. It is a technique used in multi-antenna broadband systems since it significantly reduces the complexity of the receiver by providing orthogonal sub-channels.
Multipath effects in OFDM are greatly reduced by adding a cyclic prefix (CP) to each OFDM symbol. The CP acts as a guard interval between successive OFDM symbols. In conventional OFDM systems, a fixed-length CP is used. If the channel delay spread is less than or equal to the CP, inter-symbol interference (ISI) is prevented. When the receiver is moving from one environment to another, this condition may not be met, ISI will occur, and the system will be impaired.
The phase noise in OFDM can cause inter-channel-interference (ICI). The use of a cyclic prefix also reduces ICI.
A drawback to OFDM and the use of a fixed-length CP is the reduction in rate due to the CP overhead. If the fixed length CP is not long enough, system performance deteriorates. If the CP length is too long, spectrum efficiency is reduced.
Zhang et al., “A novel OFDM transmission scheme with length-adaptive Cyclic Prefix”, J Zhejiang University SCI, 2004, describes a technique applied to a mobile wireless environment to create a variable length CP in order to counter the aforementioned drawbacks of fixed CP length. The paper describes a system where the CP length is reduced when delay spread is small thus reducing overhead. Conversely, the CP length is increased when delay spread is high and ISI eliminated. The disclosed technique utilizes known symbols in the preamble of each packet transmission or pilot sub-carriers to estimate the channel parameters. The channel parameters are used to determine the RMS delay spread and the resulting CP length needed. The technique is inefficient in slowly changing environments because of the preamble overhead or the pilot symbols present in each transmission. Accuracy in the determination of the channel parameters decreases as the number of pilot symbols is reduced to increase spectrum efficiency. In order to compute the delay spread accurately, a long sequence is needed, but the continual transmission of a long sequence uses bandwidth inefficiently.